Dry Season Streamflow Research Articles

The response in water yield to the thinning of Pinus radiata, Pinus patula and Eucalyptus grandis plantations

The paired catchment method was used to test for the effects of thinning on the water yield in three afforested catchments in South Africa, namely, Biesievlei, Jonkershoek, 98% afforested with Pinus radiata (three thinnings); Westfalia catchment D, 100% afforested with Eucalyptus grandis (two thinnings); and Cathedral Peak CII that was 74% afforested with Pinus patula (one thinning). During and after two separate thinnings, each of which removed roughly one third of the stems in a maturing P. radiata plantation in the Biesievlei catchment, annual streamflow increased by between 10 and 71% (19–99 mm). These increases persisted for three and two years after the thinning, respectively. A final thinning in the same catchment removed only 22% of stems at an age of 28 yr. The following years (1977 and 1978) were wetter than average, and reductions in annual streamflow of 26 and 55% were recorded in these two years. At Westfalia catchment D and Cathedral Peak CII, the hydrological trends were entirely dominated by the rapidly declining streamflow caused by the developing E. grandis and P. patula plantations respectively. Any savings in water use that may have resulted from the thinning of these plantations were insufficient to affect the downward trend in annual streamflow. Thinnings may have had a minor effect of delaying or reducing the desiccation of these catchments but such effects could not be assessed due to natural variability and the limited resolution of the paired catchment method. The trends in total water yield from the catchments were generally mirrored in the dry season streamflow, and there were no strong indications that thinning effects are linked to a particular season.

Soil water storage and groundwater behaviour in a catenary sequence beneath forest in central Amazonia. II. Floodplain water table behaviour and implications for streamflow generation

Abstract. Valley floor groundwater level data collected during the ABRACOS project (Gash et al. 1996), and published streamflow data from small forested catchments in geomorphologically similar areas nearby have been analysed to improve the understanding of the processes of streamflow generation. Early in the wet season, the floodplain water table is typically at 0.8 m depth, or less, and receives only local, vertical recharge. Large storms may create a groundwater ridge beneath the floodplain, temporarily creating a gradient in the direction of the hilislope. Later in the wet season, floodplain water levels are controlled primarily by the discharge of groundwater which maintains the dry season streamflow. The groundwater is recharged by deep drainage from beneath the plateau and slope areas once the dry season soil water deficit has been overcome. In the late wet season, the water level is almost at the floodplain surface and may create seeps on the lower slopes in very wet years. For the period 1966-1989, the recharge was estimated to range from 290 mm to 1601 mm with a mean of 1087 mm. Published data show that baseflow is 91% of annual runoff. Stormflow is generated on the floodplain, and water table recessions after rainfall events show that the runoff response depends on the depth to the water table. These results are from areas with deeply weathered and permeable soils; in areas of Amazonia with shallower soils, the predominant flow generation processes will differ (Elsenbeer and Lack, 1996).

Catastrophic Events Reveal the Dynamic Nature of Salt-Marsh Vegetation in Southern California

Recent hydrological disturbances, including flooding, dry-season streamflow, and drought, greatly altered coastal wetland habitats in sourthern California. At Tijuana Estuary, a six-year study of salt-marsh vegetation patterns during these rare conditions documented substantial temporal variability in plant growth and distribution. Important to cordgrass (Spartina foliosa Trin.) dynamics were the amount and timing of streamflows, which reduced soil salinity and alleviated stresses on plant growth. Poorest growing conditions occurred in 1984 when both river and tidal flows were lacking; soils had low moisture and extreme salinities (avg.=104‰ in September). Plant stress was documented in 1984 as high mortality (62% fewer stems than in 1983) and reduced height (19% less than in 1983). Cordgrass height was greatest in 1980 following winter flooding (20% increase over 1979); densities were greatest in 1983 with summer freshwater influxes (60% increase over 1982). A carbon allocation model is proposed to explain the varied responses.

Factors Influencing Standing Crops and Survival of Juvenile Salmon at Barrows Stream, Maine

Biological and physical variables influencing survival and standing crops of juvenile salmon are treated by regression analyses to determine those variables of greatest significance. Stream flow in dry seasons as indicated by rainfall proved to be the most important single factor influencing survival from age 0+ to age I+ in the presence of such other variables as standing crops of other fishes and water temperatures. A Chi-square test indicated that the various species of fish present in Barrows Stream during the 6-year study reacted to electrofishing as though they were a single species. Indications are that construction of small water storage dams at appropriate sites in the headwaters of salmon nursery areas could increase survival and growth rate of juvenile salmon occupying the nurseries.